1. Field of the Invention
The present invention relates to a switching regulator for following an output voltage to change an amount of slope correction.
2. Related Background Art
As for a conventional current mode step-down switching regulator, for example, a circuit having a configuration as shown in the form of a block diagram in FIG. 3 is known.
A switch 107 serves to supply therethrough an input voltage VIN to a coil 109. An error amplifier 101 serves to amplify a difference between a voltage which is obtained by dividing an output voltage VOUT with resistors 110 and 111, and a reference voltage VREF supplied from a reference voltage source 100.
A signal which is obtained by subtracting a correction ramp wave outputted from a slope correcting circuit 102 from an output signal of the error amplifier 101 in a subtracter 103 is inputted to an inverting input terminal of a comparator 104. The correction ramp wave outputted from the slope correcting circuit 102, as shown in the figure, has a shape of a saw-tooth-wave which is synchronous with a period of an oscillator 105.
A voltage signal into which information of a current caused to flow through the switch 107, or information of a current caused to flow through the coil 109 is converted is inputted to a non-inverting input terminal of the comparator 104. A current caused to flow through the respective constituent elements is usually detected using a sense resistor which is operatively connected in series with the switch 107 or the coil 109. Thus, a voltage signal having a value which is proportional to the current caused to flow through the switch 107 or the coil 109 is inputted as voltage information to the non-inverting input terminal of the comparator 104.
A level of an output signal of the error amplifier 101 is increased when a level of the output voltage VOUT is low. Thus, in order to make transition of a level of an output signal of the comparator 104 from L to H, a voltage signal having a larger value needs to be applied to the non-inverting input terminal of the comparator 104. That is, when the level of the output voltage VOUT is low, a larger current is caused to flow through the switch 107 or the coil 109 to thereby invert the level of the output signal of the comparator 104. The output signal of the comparator 104 is inputted to a reset terminal R of an SR-latch 106.
The oscillator 105 is operatively connected to a set terminal S of the SR-latch 106. Then, pulses having a fixed period, as shown in the figure, are outputted from the oscillator 105. An output terminal Q of the SR-latch 106 is operatively connected to the switch 107. Then, when a signal level at the output terminal Q of the SR-latch 106 goes H, the switch 107 is turned ON.
In the above-mentioned current mode step-down switching regulator, for example, a current caused to flow through the coil connected between the input terminal and an output terminal for the voltage may oscillate in some cases. In order to prevent this current oscillation, a slope correction becomes necessary. In order to suitably prevent the current oscillation, a suitable amount of slope correction needs to be carried out for a decreasing rate (slope) of a coil current.
However, when the output voltage VOUT of the current mode step-down switching regulator is made variable, in the conventional current mode step-down switching regulator shown in FIG. 3, there arises a problem in that a decreasing rate of the coil current of the coil 109 is changed as the output voltage VOUT is changed, whereas an amount of slope correction becomes unsuitable because an increasing rate (slope) of the correction ramp wave as an amount of slope correction outputted from the slope correcting circuit is fixed.